Long-term increases in soil carbon due to ecosystem fertilization by atmospheric nitrogen deposition demonstrated by regional-scale modelling and observations.

Fertilization of nitrogen (N)-limited ecosystems by anthropogenic atmospheric nitrogen deposition (Ndep) may promote CO2 removal from the atmosphere, thereby buffering human effects on global radiative forcing. We used the biogeochemical ecosystem model N14CP, which considers interactions among C (carbon), N and P (phosphorus), driven by a new reconstruction of historical Ndep, to assess the responses of soil organic carbon (SOC) stocks in British semi-natural landscapes to anthropogenic change. We calculate that increased net primary production due to Ndep has enhanced detrital inputs of C to soils, causing an average increase of 1.2 kgCm-2 (c. 10%) in soil SOC over the period 1750-2010. The simulation results are consistent with observed changes in topsoil SOC concentration in the late 20th Century, derived from sample-resample measurements at nearly 2000 field sites. More than half (57%) of the additional topsoil SOC is predicted to have a short turnover time (c. 20 years), and will therefore be sensitive to future changes in Ndep. The results are the first to validate model predictions of Ndep effects against observations of SOC at a regional field scale. They demonstrate the importance of long-term macronutrient interactions and the transitory nature of soil responses in the terrestrial C cycle.

A dual porosity model of nutrient uptake by root hairs.

• The importance of root hairs in the uptake of sparingly soluble nutrients is understood qualitatively, but not quantitatively, and this limits efforts to breed plants tolerant of nutrient-deficient soils. • Here, we develop a mathematical model of nutrient uptake by root hairs allowing for hair geometry and the details of nutrient transport through soil, including diffusion within and between soil particles. We give illustrative results for phosphate uptake. • Compared with conventional 'single porosity' models, this 'dual porosity' model predicts greater root uptake because more nutrient is available by slow release from within soil particles. Also the effect of soil moisture is less important with the dual porosity model because the effective volume available for diffusion in the soil is larger, and the predicted effects of hair length and density are different. • Consistent with experimental observations, with the dual porosity model, increases in hair length give greater increases in uptake than increases in hair density per unit main root length. The effect of hair density is less in dry soil because the minimum concentration in solution for net influx is reached more rapidly. The effect of hair length is much less sensitive to soil moisture.

A dynamic model of nutrient uptake by root hairs.

Root hairs are known to be important in the uptake of sparingly soluble nutrients by plants, but quantitative understanding of their role in this is weak. This limits, for example, the breeding of more nutrient-efficient crop genotypes. We developed a mathematical model of nutrient transport and uptake in the root hair zone of single roots growing in soil or solution culture. Accounting for root hair geometry explicitly, we derived effective equations for the cumulative effect of root hair surfaces on uptake using the method of homogenization. Analysis of the model shows that, depending on the morphological and physiological properties of the root hairs, one of three different effective models applies. They describe situations where: (1) a concentration gradient dynamically develops within the root hair zone; (2) the effect of root hair uptake is negligibly small; or (3) phosphate in the root hair zone is taken up instantaneously. Furthermore, we show that the influence of root hairs on rates of phosphate uptake is one order of magnitude greater in soil than solution culture. The model provides a basis for quantifying the importance of root hair morphological and physiological properties in overall uptake, in order to design and interpret experiments in different circumstances.

The effects of water regime on phosphorus responses of rainfed lowland rice cultivars.

BACKGROUND AND AIMS: Soil phosphorus (P) solubility declines sharply when a flooded soil drains, and an important component of rice (Oryza sativa) adaptation to rainfed lowland environments is the ability to absorb and utilize P under such conditions. The aim of this study was to test the hypothesis that rice cultivars differ in their P responses between water regimes because P uptake mechanisms differ. METHODS: Six lowland rice cultivars (three considered tolerant of low P soils, three sensitive) were grown in a factorial experiment with three water regimes (flooded, moist and flooded-then-moist) and four soil P levels, and growth and P uptake were measured. Small volumes of soil were used to maximize inter-root competition and uptake per unit root surface. The results were compared with the predictions of a model allowing for the effects of water regime on P solubility and diffusion. KEY RESULTS: The plants were P stressed but not water stressed in all the water regimes at all P levels except the higher P additions in the flooded soil. The cultivar rankings scarcely differed between the water regimes and P additions. In all the treatments, the soil P concentrations required to explain the measured uptake were several times the concentration of freely available P in the soil. CONCLUSIONS: The cultivar rankings were driven more by differences in growth habit than specific P uptake mechanisms, so the hypothesis cannot be corroborated with these data. Evidently all the plants could tap sparingly soluble forms of P by releasing a solubilizing agent or producing a greater root length than measured, or both. However, any cultivar differences in this were not apparent in greater net P uptake, possibly because the restricted rooting volume meant that additional P uptake could not be converted into new root growth to explore new soil volumes.

The potential for nitrification and nitrate uptake in the rhizosphere of wetland plants: a modelling study.

BACKGROUND AND AIMS: It has recently found that lowland rice grown hydroponically is exceptionally efficient in absorbing NO3-, raising the possibility that rice and other wetland plants growing in flooded soil may absorb significant amounts of NO3- formed by nitrification of NH4+ in the rhizosphere. This is important because (a) this NO3- is otherwise lost through denitrification in the soil bulk; and (b) plant growth and yield are generally improved when plants absorb their nitrogen as a mixture of NO3- and NH4+ compared with growth on either N source on its own. A mathematical model is developed here with which to assess the extent of NO3- absorption from the rhizosphere by wetland plants growing in flooded soil, considering the important plant and soil processes operating. METHODS: The model considers rates of O2 transport away from an individual root and simultaneous O2 consumption in microbial and non-microbial processes; transport of NH4+ towards the root and its consumption in nitrification and uptake at the root surface; and transport of NO3- formed from NH4+ towards the root and its consumption in denitrification and uptake by the root. The sensitivity of the model's predictions to its input parameters is tested over the range of conditions in which wetland plants grow. KEY RESULTS: The model calculations show that substantial quantities of NO3- can be produced in the rhizosphere of wetland plants through nitrification and taken up by the roots under field conditions. The rates of NO3- uptake can be comparable with those of NH4+. The model also shows that rates of denitrification and subsequent loss of N from the soil remain small even where NO3- production and uptake are considerable. CONCLUSIONS: Nitrate uptake by wetland plants may be far more important than thought hitherto. This has implications for managing wetland soils and water, as discussed in this paper.

Isotopic discrimination of zinc in higher plants.

* The extent of isotopic discrimination of transition metals in biological processes is poorly understood but potentially has important applications in plant and biogeochemical studies. * Using multicollector inductively coupled plasma (ICP) mass spectrometry, we measured isotopic fractionation of zinc (Zn) during uptake from nutrient solutions by rice (Oryza sativa), lettuce (Lactuca sativa) and tomato (Lycopersicon esculentum) plants. * For all three species, the roots showed a similar extent of heavy Zn enrichment relative to the nutrient solution, probably reflecting preferential adsorption on external root surfaces. By contrast, a plant-species specific enrichment of the light Zn isotope occurred in the shoots, indicative of a biological, membrane-transport controlled uptake into plant cells. The extent of the fractionation in the shoots further depended on the Zn speciation in the nutrient solution. * The observed isotopic depletion in heavy Zn from root to shoot (-0.13 to -0.26 per atomic mass unit) is equivalent to roughly a quarter of the total reported terrestrial variability of Zn isotopic compositions (c. 0.84 per atomic mass unit). Plant uptake therefore represents an important source of isotopic variation in biogeochemical cycling of Zn.

Comparative kinetic analysis of ammonium and nitrate acquisition by tropical lowland rice: implications for rice cultivation and yield potential.

Nitrogen limitation compromises the realization of yield potential in cereals more than any other single factor. In rice, the world's most important crop species, the assumption has long been that only ammonium-N is efficiently utilized. Consequently, nitrate utilization has been largely ignored, although fragmentary data have suggested that growth could be substantial on nitrate. Using the short-lived radiotracer 13 N, we here provide direct comparisons of root transmembrane fluxes and cytoplasmic pool sizes for nitrate- and ammonium-N in a major variety of Indica rice (Oryza sativa), and show that nitrate acquisition is not only of high capacity and efficiency but is superior to that of ammonium. We believe our results have implications for rice breeding and molecular genetics as well as the design of water-management and fertilization regimes. Potential strategies to harness this hitherto unexplored N-utilization potential are proposed.

Root-induced iron oxidation, pH changes and zinc solubilization in the rhizosphere of lowland rice.

Rice plants (Oryza sathiva L., cv. IR34) were grown with their roots sandwiched between cylinders of an anaerobic low-Zn Mollisol. After periods of root-soil contact of up to 12 d (total plant age c. 28 d) the profiles of different Zn fractions, reduced and oxidized Fe, and pH in the soil near the root plane' were determined. The concentration of easily plant-extractable Zn in the soil (measured by extraction in I M KCl) was negligible, and so it was necessary for the plants to induce changes in the soil to solubilize Zn. After 6 d, there was a substantial accumulation of Zn associated with organic matter and amorphous ferric hydroxide within 4-5 mm of the root plane. Over the next 6 d, the accumulation continued but there was a substantial depletion of the accumulated fractions within 2 mm of the root plane. The zones of accumulation and depletion coincided with zones of Fe(III) accumulation and soil acidification in which the pH decreased from the bulk soil value of 7.3 by over 0.2 pH units; i.e. a two-fold increase in H+ concentration, The acidification was the result of H+ released from the roots to balance excess intake of cations over anions, and H+ generated in the oxidation of Fe(H) by root-released O2 . At the high pH and CO., pressure of the experimental soil (7.3 and c. 0.9 kPa. respectively), soil acidity diffusion is fast and consequently the pH drop at the root surface was small. The rate of Fe oxidation peaked before 6 d, but the acidification and Zn accumulation continued beyond 6 d unabated. It is concluded that Fe oxidation released Zn from highly insoluble fractions, and that this Zn was re-adsorbed on Fe(OH)3 and on organic matter in forms that were acid-soluble and therefore accessible to the plants.

Root-induced solubilization of phosphate in the rhizosphere of lowland rice.

Lowland rice (Oryza sativa L., cv IR74) was grown in cylinders of a P-deficient reduced Ultisol separated into upper and lower cells by a fine nylon mesh so that the roots formed a planar layer above the mesh. This enabled changes in soil P fractions and other root-induced changes in the soil near the root plane to be measured. In both P-fertilized and unfertilized soil, the quantity of readily plant-available P was negligible in comparison with the quantity of P extracted by the plants, and the plants therefore necessarily induced changes in the soil so as to solubilize P. After 6 wk of growth, 90 % of the P taken up was drawn from acid-soluble pools. The remainder was from an alkali-soluble inorganic pool which was on balance depleted, although its concentration profile contained zones of accumulation corresponding to zones of Fe(III) accumulation. There was also a small accumulation of alkali-soluble organic P. There were no changes in the more recalcitrant soil P pools. The zone of P depletion was 4-6 mm wide, increasing with P addition, and coincided with a zone of acidification in which the pH fell from near 6 in the soil bulk to less than 4 near the roots. The acidification was due to H+ generated in oxidation of Fe2+ by root-released O2 , and to H+ released from the roots to balance excess intake of cations over anions. With increasing P deficiency there were increases in the ratio of root: shoot d. wt; the ratio of shoot d. wt to total P in the plant; the excess intake of cations over anions per unit plant d. wt and corresponding release of H+ to the soil; and the quantity of Fe oxidized per unit plant d. wt and corresponding release of H+ to the soil. Independent, in vitro measurements confirmed that acid addition increased the P concentration in the soil solution and the quantity of P that could be desorbed per gram of soil. A mathematical model of the diffusion of acid away from the roots, acid reaction with the soil in solubilizing P, and the diffusion of P back to the absorbing roots showed that, under the conditions of the root-plane experiments, solubilization by acidification accounted for at least 80% of the P taken up in both P-fertilized and unfertilized soil, but that less than 50% of the P solubilized could be taken up by the roots.

Root-induced iron oxidation and pH changes in the lowland rice rhizosphere.

Measurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice (Oryza sativa L.) roots are reported. Initially 11-d-old plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a well-defined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH4 + . But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO2 uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed.